FOR U01AI131470 Tolerance of kidney allografts has been achieved in non-human primates (NHPs) and in humans using a combination of nonmyeloablative conditioning and donor bone marrow transplantation (DMBT) that results in transient donor chimerism. However, similar conditioning failed to induce tolerance in heart recipients despite comparable levels of chimerism. The reasons for this organ-specific difference are not clear. However, it is clear that all transplanted organs are not created equally. Not only does the strength of the immune response to a particular organ vary with the organ transplanted but the nature of response itself, rejection versus tolerance, varies from organ to organ. It is well known that some organs, such as kidney and liver, are tolerance-prone while others, such as heart and lung, are tolerance-resistant. In earlier studies using miniature swine, we took advantage of the tolerogenicity of kidney allografts and by cotransplanting donor kidneys with heart allografts, achieved long-term stable tolerance of heart allografts which, if transplanted alone, would have rejected acutely. We have now extended those findings to NHPs by combining donor kidney cotransplantation with a mixed chimerism protocol that induces transient donor chimerism. This protocol is the first to achieve long-term tolerance of MHC mismatched heart allografts in NHPs. Importantly, every recipient that successfully completed its protocol achieved indefinite allograft survival and did so without evidence of cardiac allograft vasculopathy (CAV). Kidney-induced cardiac allograft tolerance (KICAT) is made even more compelling by its consistency across 1) different species (mouse, swine, NHP), 2) different histocompatibility barriers, and 3) different tolerance protocols. These findings suggest that immune mechanisms exist which are capable of inducing tolerance to any organ. Elucidating those mechanisms would lead to strategies that extend tolerance to all allograft recipients. To achieve that goal we propose to 1) investigate novel mechanisms that may explain the tolerogenicity of kidney allografts, 2) evaluate unique regulatory mechanisms the may facilitate KICAT, and 3) test alternative strategies that obviate the need for donor kidney cotransplantation in achieving heart allograft tolerance. Our specific aims are 1) to determine if MHC crossdressing via donor microvesicles (exosomes) or Treg-rich organized lymphoid structures (TOLs) underlie the fundamental differences observed in host alloresponses to kidney allografts, 2) to evaluate the contribution of Tr1 cells and a novel CD8+CD20+ B cell to kidney-induced cardiac allograft tolerance, and 3) to determine if combined low dose IL-2/anti-IL-6R therapy or donor thymus cotransplantation will successfully substitute for donor kidney transplantation and achieve tolerance in recipients of isolated heart allografts.